/Engine/Source/Runtime/Slate/Public/Framework/MultiBox/MultiBoxBuilder.h
void AddSubMenu ( const [FUIAction](API\Runtime\Slate\Framework\Commands\FUIAction) & UIAction, const [TSharedRef](API\Runtime\Core\Templates\TSharedRef)< [SWidget](API\Runtime\SlateCore\Widgets\SWidget) > Contents, const FNewMenuDelegate & InSubMenu, const bool bInShouldCloseWindowAfterMenuSelection )